A bonded magnet is produced by kneading a composition containing a magnet powder, a binder component such as an organic resin, and an additive such as a strengthening agent, a plasticizer, or a lubricant in combination using an extruder or the like, processing the kneaded product into pellets or the like, followed by injection molding, compression molding, or extrusion. In particular, a magnet that is produced by injection molding with the use of a thermoplastic resin such as a polyamide resin or a polyphenylenesulfide resin as a binder has high dimensional accuracy and requires no post-processing, and thus is advantageous in that the cost required for producing magnet can be reduced.
A composition for bonded magnets containing 80 mass % or more of a magnet powder and a thermoplastic resin as a binder is processed by injection molding or extrusion into a bonded magnet.
A polyamide resin, a polyphenylenesulfide (PPS) resin or the like is used as a thermoplastic resin binder, but a polyamide resin is more preferred in terms of mechanical strength, weather resistance, and the like. A resin composition containing a polyamide resin and a polyamide elastomer used therein has been proposed (for example, see Patent documents 1 and 2).
Moreover, it has also been proposed to knead carbon fibers as a strengthening agent into a composition for bonded magnets produced by kneading a magnet powder and an organic resin in order to improve the mechanical strength of a bonded magnet (for example, see Patent documents 3 and 4).
As described above, a bonded magnet is produced by injection molding or extrusion, and is often used in combination with a metal component for bonding with an adhesive, insert molding, and the like. However, a metal component and a bonded magnet material have different linear expansion coefficients, so that when a thermal shock test is conducted, the bonded magnet is cracked in many cases.
Furthermore, regarding the composition for bonded magnets, the higher the blending ratio of a magnet powder, the higher the magnetic properties as an injection-molded magnet. Hence, the blending ratio of a magnetic powder is preferably increased. However, the higher the blending ratio of a magnet powder, the lower the fluidity of the composition, which makes processing by injection molding difficult and may cause poor appearance such as a weld upon injection molding. An additive is generally added in order to improve fluidity. The fluidity is improved by the addition of an additive, however, this often adversely affects the strength, such that the thus molded bonded magnet becomes easily cracked in a thermal shock test. Moreover, the production of an integrally molded component by combining with a metal component using an adhesive is problematic in that the additive lowers the bonding strength between the bonded magnet and the metal component, making the addition of a lubricant difficult.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2004-352792
Patent Document 2: PCT International Publication No. WO2008/123450
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2005-072240
Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2010-251545